Nowadays, with popularity of intelligent mobile terminals and development of wireless internet services, mobile data services tend to be booming, which brings unprecedented challenges for wireless networks. To meet mobile broadband requirements, currently 4G Long Time Evolution (LTE) networks adopt various technologies such as carrier aggregation (CA), large scale MIMO, Coordinated Multi-Point (CoMP), etc. to improve wireless network capacity, and to further improve frequency spectrum effectiveness, researches for small cells are being actively pushed.
However, since the range of carrier frequencies currently being used for wireless communication is 700 MHz˜2.6 GHz, and bandwidth of available frequency spectrum resources is still very limited, new technologies such as CA, large scale MIMO, CoMP, etc. only improve peak communication rate of a single subscriber or frequency spectrum effectiveness of a certain cell to some extent, and in the given range of the frequency spectrum resources, improvements to wireless network capacity are limited, which maynot meet the increasing capacity requirement. Especially, it is predicted that compared to what it is now, by year 2020, the wireless network capacity may grow 1000 times, and the capacity requirement maynot be met by only improving the frequency spectrum efficiency. While with the heterogeneous network architecture being put forward, though deployment technologies of more intensive small cells are still developing and the frequencies are reused by way of increasing low power access points to improve the capacity, subjected to influences by factors such as base station site selection, device installation, backhaul network construction, etc., costs of realizing small cells are very high, and to increase the capacity, simply increasing the number of small cell sites is not enough. To make the increase of the capacity and the number of small cells be a linear relation, the most important thing is to solve the co-channel interference caused by reusing the frequencies, and being heavily dependent on high level interference elimination technologies is not what is expected. So only by using the small cell technology, it is difficult to meet the increasing wireless data service requirement. Obviously, multi-dimensional capacity improvement methods are effective ways to meet future requirements. That is, an overall capacity requirement may be realized by way of using more small cells, improving the frequency efficiency, improving the frequency spectrum utilization, and introducing more frequency spectrum resources.
However, the frequency spectrum resources are rare and frequency spectrum resources of low frequency bands are very congested (e.g., 700 MHz˜2.6 GHz currently being used in wireless communication), and there is a tendency of developing higher frequency bands. To meet the requirement of the predicted 1000 times capacity increase by year 2020, the gap of the frequency spectrum is still large. In this circumstance, people pay more and more attentions on high frequency band communication technologies. Millimeter wave communication technology is a most representative high frequency band communication technology, and generally millimeter wave has a frequency spectrum from 26.5 GHz to 300 GHz. It may be seen that, besides of frequency bands of 57 GHz˜64 GHz and of 164 GHz˜200 GHz that are susceptible to oxygen and water losses, the millimeter wave is able to provide bandwidth up to 230 GHz, which is more than 100 times of that of the frequency spectrum resources currently being used, and may meet the wideband wireless data service requirements more better. This is the reason why the millimeter wave communication is widely concerned.
However, the millimeter wave has weakness that it maynot evade. First, frequencies of the millimeter wave are higher than carrier frequencies currently being used for wireless communication. According to a classical free-space path loss rule, i.e., LFSL=32.4+20 log 10f+20 log 10R, where LFSL is free-space path loss represented by dB, f is a carrier frequency, R is a distance between a transmitter and a receiver, the free-space path loss that the millimeter wave with the lowest frequency (26.5 GHz) confronts is 20 dB higher than the path loss that the highest frequency carrier (2.6 GHz) for wireless communication confronts. Therefore, if the millimeter wave is used to cover wireless communication cells, the weakness will dramatically influence the coverage of the millimeter wave cells. Besides of this, in an actual wireless communication environment, the oxygen and water in the air will absorb energy of the millimeter wave, which further influences the propagation distance of the millimeter wave.
An ideal method that solves the above defect of the millimeter wave is to combine the large scale MIMO technology and the beamforming technology. The method may concentrate energy of the millimeter wave on a very narrow weave beam so that propagation of the millimeter wave has very strong directivity, and during point-to-point downlink transmission, it may guarantee better coverage. However, for a real wireless communication cell, besides of the point-to-point downlink transmission, there is other downlink transmission dedicated for multiple users, for example, broadcast messages of synchronization channels, public control channels, and cells, and transmission of the information will aim at multiple users, but, generally, different users may be distributed on different locations of a cell, and a narrow band beam realized by beamforming may only be directed to one direction, therefore, the broadcast messages of the synchronization channels, public control channels and cells may only be transmitted by using a traditional way so as to give consideration to the users in the whole cell. Compared to that of the point-to-point downlink transmission that uses the beamforming technology, energy of wireless signals that carry the broadcast messages of the synchronization channels, public control channels and cells is more diffused so that under a same transmission power condition, their coverage will be smaller than that of the point-to-point downlink transmission signals. Realization of beamforming relies on estimate of radio channels, while estimate of radio channels is generally realized by using a reference signal, but during an actual communication process, during transmission of messages at an initial stage of establishing a connection between a wireless terminal and a base station, e.g., a first downlink message, the base station is unable to obtain information of radio channels, which will affect coverage of wireless signals that carry the messages, however, the messages are critical for establishing a wireless link between the terminal and the base station.
In addition, due to restriction on sizes of mobile terminals, generally, large scale MIMO may not be performed on a terminal side. That is, scale of an antenna array on a terminal side is far smaller than scale of an antenna array on a base station side. Therefore, during uplink transmission, concentration degree of beams will be smaller than that of beams during point-to-point downlink transmission, and there is an asymmetric problem existing in coverage of uplink and downlink signals.